Variable inductance arrangement



Nov. 24, 1942. w. VAN B. ROBERTS 2,302,393

VARIABLE INDUCTANCE ARRANGEMENT Filed Sept. 29, 1959 2 Sheets-Sheet 1 0Eyd SOURCE F IRE CURRENT 0K WLY VARY ('0 0L CURRENT ENTOR WA L TER B.ROBERTS ATTORNEY Nov. 24, 1942. w V ROBERTS 2,302,893

VARIABLE INDUCTANCE ARRANGEMENT I Filed Sept. 29, 1939 2 Sheets-Sheet 2.9 4 r0 PUSH-PULL Rf. SELECTOR .12 A F. GRIDS AND AMPL/F/ER AAA AAAA vvyvvv qllp FREQl/fA/CY CONVERT 5k AL 1. F. AMPL lF/ER v INVENTOR WALTERVAN B ROBERTS ATTORNEY Patented Nov. 24, 1942 2,302,893 VARIABLEINDUCTANCE ARRANGEMENT Walter van B. Roberts, Princeton, N. J., assignorto Radio Corporation of America, a corporation of Delaware ApplicationSeptember 29, 1939, Serial No. 297,032

1 Claim.

It has been proposed to vary the inductance of a coil having a magneticcore by superposing on said core a controlling flux which is adjustablein accordance with the inductance desired. However, if the coil carryingthe controlling current is coupled to the radio frequency winding losseswill be produced in the radio frequency winding. The present inventionprovides a construction which permits both the radio frequency coil andthe control current winding to produce flux in a common portion of theirmagnetic circuits without thereby introducing any coupling between thetwo coils provided that the one coil carries radio frequency current andthe other coil only direct or slowly varying current.

In the drawings:

Fig. 1 shows a variable inductance embodying the invention,

Fig. 2 is a modification,

Fig. 3 shows a radio receiver circuit employing the variable inductancedevice at the antenna circuit,

Fig. 4 shows a superheterodyne receiver employing the variableinductance device for automatic frequency control.

Fig. 1 shows one form of the invention where l represents a highfrequency core made up of a material such as powdered iron and where thedotted lines 2 bearing arrows indicate the magnetic lines of forceproduced by current in the radio frequency coil 3. The core I isprovided with a cylindrical copper shield 4 which surrounds the magneticcore 3 and this shield prevents radio frequency variations of magneticflux from getting outside of the shield. Member 5 which is made ofordinary iron has mounted thereon control windings 6 and I. Thisarrangement forms a magnetic circuit which is completed by the magneticmaterial inside the shield 4. Typical lines of flux are shown by lines 9and Ill. Due to the fact that the control flux represented by lines 9and I0 is at most only slowing varying the copper shield 4 isineffective to prevent the control flux from passing through the shieldand acts merely like a small air gap. It will be noted that in additionto its primary purpose of preventing the radio frequency field fromlinking the control winding or the solid iron portion of the controlcircuit, the shield also acts in the usual way to prevent couplingbetween the radio frequency coil and other coils adjacent thereto. a

It will be noted also that a small air gap H is left in the radiofrequency magnetic circuit so that the control flux will pass mainlythrough the central portion of the radio frequency core I where itseffect is the most pronounced. Windings 6 and I are connected to asuitable source of control current through terminals 8.

Fig. 2 shows another arrangement operating similarly in principle to thearrangement shown in Fig. 1 except that the radio frequency coil I2 andcore l3 are elongated and the control winding I4 are wrapped upon theshield l5. An air gap may be left in the radio frequency magneticcircuit if desired but in this case an air gap will not increase theflux through the interior of the radio frequency coil.

Fig. 3 shows the application of a controlled inductance of the typedescribed to the input circuit of a radio receiver for the purpose ofpreventing overloading of the first tube. The antenna circuit is isnormally unbalanced in the presence of weak signals but when the signalsare strong enough to produce a predetermined detector input current thispredetermined current flowing through the control winding 21 of theantenna coil l8 makes its inductance decrease to a point that will bringthe bridge circuit l1, l8 and IS in the antenna system to balance, thuspreventing any voltage reaching the first tube 24. Obviously, thedetector input can never quite reach this value and hence the input tothe first tube 24 can never exceed a predetermined amount. In additionto this type of automatic control of input voltage the rectifled voltagedeveloped across resistor 29 may be also applied to one or more of thegrids of the amplifier tubes shown generally at 30 to control the gainthereof in the usual way.

Fig. 4 shows the application of a coil in accordance'with the inventionto autcmatic frequency control. The coil is used in the oscillator 30 ofa superheterodyne whose frequency is approximately adjusted by thevariable condenser 3|. If the frequency is not quite correct a voltagewill be developed by the discriminator circuit 32 and thus produce aflux in one direction or the other in the control magneto circuit 33according to the direction of the misadjustment. A third winding 34energized to a constant amount by a battery 35 is employed with theresult that a flux in one direction produced by the discriminatorincreases the total flux in the control core 36 while the flux in theopposite direction decreases the total flux. Thus the effectiveinductance of the oscillator coil 31 is increased or decreased accordingas the mistuning is in one sense or the other. The action in otherrespects is identical with that oi known A. F. C. circuits employing aso-called reactance tube for controlling the oscillator frequency. Theadvantage in the present arrangement lies in the elimination of theextra tube.

I claim:

A compact variable inductance unit comprising a cylindrical radiofrequency term-magnetic core, a single radio frequency coil woundcoaxially along solely said core, a non-magnetic 10

